Mystery Boxes are one of the two options available to buy in the Shop, the other being Packages. These items are usually bought with Realm Gold , but every once in a while a Mystery Box will be offered for Fame .
Mystery boxes offer a variety of equipment, consumables, skins, cloths, and miscellaneous items for a player to buy, with an element of chance. Purchasing one will randomly give you an item from the possible contents of the box. In some boxes you can receive more than one item per purchase.
Keep in mind that the items displayed on the top of a particular box have the lowest chance of dropping, unless stated otherwise, and the items listed at the bottom of the box have the highest chance.
Once purchased, the item(s) won will be sent to your Vault’s Gift Chest.
For current Mystery Boxes, check this page for details.
Below is an example of the Mystery Boxes screen within the Shop, alongside the possible contents of one box.